The circulator is basic to both theory and practical applications of nonreciprocity in electromagnetic systems. Microwave junction circulators have become widely employed in waveguide and coaxial versions and, in recent years, in planar stripline embodiments due to the exploitation of planar, miniature, and integrated circuits. The three-port ring-network circular was introduced as a theoretic formulation in 1965.
Weiss, J. A., "Circulator Synthesis"IEEE Trans. MTT 13, 38-44 (Jan., 1965) PA1 Ewing, S. D. and Weiss, J. A., "Ring Circulator PA1 U.S. Pat. No. 3,304,519, issued Feb. 14, 1967 to Weiss, J. A., incorporated herein by reference.
experimental verification in 1967:
Theory, Design and Performance"IEEE Trans. MTT 15, 623-628 (Nov., 1967)
and issued as a patent in 1967:
The specific embodiment considered in the 1965 study was a ring comprising three identical non-reciprocal phase shifters connected by three identical, symmetrical, reciprocal T-junctions, constituting a three-port junction circulator. Computations performed for a range of examples demonstrated that circulation is achievable with unexpectedly small requirements for nonreciprocity in the sectors between T-junctions. At the time of the first publications, the potential advantages of the ring network were not apparent, as compared with the supposed disadvantages of loss and complexity suggested by those initial designs. In addition, exploitation of the concepts of planar circuits, integration, and miniaturization were in an infant stage of development. For these reasons, the theory proposed in 1965 has received only slight attention from of the microwave non-reciprocal device community.
The ring network circulator disclosed in 1965 was dismissed as "significantly large and more complicated" than a lumped-element circulator because the "ring circulator uses three delta connected non-reciprocal phase shifters": Knerr, R. H., "A Lumped-Element Circulator Without Crossovers", IEEE Trans MTT, Vol. 22, pp. 544-548 (May, 1974). In another study, three meanderline non-reciprocal phase shifters were combined in a ring by three T-junctions, and the combination was deposited on a ferrite disk: Sherman M., "Stripline Ferrite Devices", Syracuse University Research Corporation--Special Projects Laboratory, Tech Rep. No. RADC-TR-68-71 (Jan, 1968), AD No. 827769. The study resulted in a circulator having unfavorable characteristics: "2 dB insertion loss and a bandwidth of approximately 2%", concluding that "It appears doubtful that bandwidth greater than 10% can be obtained from the ring circulator."